Mobile communication system, transmission-side terminal, mobile communication method and processor

ABSTRACT

The mobile communication system directly performs user-data communication between a plurality of radio terminals without involvement of a radio base station. The user-data communication directly performed between the plurality of radio terminals is performed by using a part of radio resources assigned to the mobile communication system. The plurality of radio terminals comprise: a transmission-side terminal that transmits the user data; and a reception-side terminal that receives the user data. The reception-side terminal comprises: a reception-side transmission unit that transmits a transmission acknowledgement signal indicating whether the user data transmitted from the transmission-side terminal has been able to receive or not. The reception-side transmission unit transmits the transmission acknowledgement signal to the radio base station.

TECHNICAL FIELD

The present invention relates to a mobile communication system in whichuser-data communication is directly performed between a plurality ofradio terminals, a transmission-side terminal that transmits user datato a reception-side terminal in the mobile communication system, amobile communication method used in the mobile communication system, anda processor causing the transmission-side terminal to perform a process.

BACKGROUND ART

In recent years, there has been proposed a technology in whichcommunication of user data (data of User-Plane) is directly performedbetween a plurality of radio terminals without involvement of a radiobase station (D2D communication). The user-data communication directlyperformed between the plurality of radio terminals, is performed byusing a part of radio resources assigned to a mobile communicationsystem. However, in the D2D communication, communication of control data(data of C-Plane) is performed via the radio base station, similarly toa conventional mobile communication system.

In general, a transmission acknowledgement signal (Ack/Nack signal),which indicates whether data transmitted from a transmission-sideterminal has been able to be received, is transmitted from areception-side terminal to the transmission-side terminal, so thatretransmission control is performed.

Meanwhile, in the D2D communication, the radio base station is able toreceive the user data communicated between the plurality of radioterminals. Consequently, in the D2D communication, it is possible toconsider various methods as a method for the retransmission control.

SUMMARY OF INVENTION

A mobile communication system according to a first feature directlyperforms user-data communication between a plurality of radio terminalswithout involvement of a radio base station. The user-data communicationdirectly performed between the plurality of radio terminals is performedby using a part of radio resources assigned to the mobile communicationsystem. The plurality of radio terminals comprise: a transmission-sideterminal that transmits the user data; and a reception-side terminalthat receives the user data. The reception-side terminal comprises: areception-side transmission unit that transmits a transmissionacknowledgement signal indicating whether the user data transmitted fromthe transmission-side terminal has been able to receive or not. Thereception-side transmission unit transmits the transmissionacknowledgement signal to the radio base station.

A transmission-side terminal according to a second feature transmitsuser data to a reception-side terminal, of a plurality of radioterminals, in a mobile communication system in which the user-datacommunication between the plurality of radio terminals is directlyperformed without involvement of a radio base station. The user-datacommunication directly performed between the plurality of radioterminals is performed by using a part of radio resources assigned tothe mobile communication system. The transmission-side terminalcomprises: a transmission-side reception unit that receives atransmission acknowledgement signal indicating whether thereception-side terminal has been able to receive the user data. Thetransmission-side reception unit directly receives the transmissionacknowledgement signal from the reception-side terminal and receives thetransmission acknowledgement signal via the radio base station.

A mobile communication method according to a third feature is used in amobile communication system in which user-data communication between aplurality of radio terminals is directly performed without involvementof a radio base station. The user-data communication directly performedbetween the plurality of radio terminals is performed by using a part ofradio resources assigned to the mobile communication system. Theplurality of radio terminals comprise: a transmission-side terminal thattransmits the user data; and a reception-side terminal that receives theuser data. The mobile communication method comprises: a step A oftransmitting, by the reception-side terminal, a transmissionacknowledgement signal indicating whether the user data transmitted fromthe transmission-side terminal has been able to receive or not. The stepA comprises: a step of transmitting the transmission acknowledgementsignal to the radio base station.

A processor according to a fourth feature causes a transmission-sideterminal to perform a process, the transmission-side terminaltransmitting user data to a reception-side terminal, of a plurality ofradio terminals, in a mobile communication system in which the user-datacommunication between the plurality of radio terminals is directlyperformed without involvement of a radio base station. The user-datacommunication directly performed between the plurality of radioterminals is performed by using a part of radio resources assigned tothe mobile communication system. The processor causes thetransmission-side terminal to perform a process A of receiving atransmission acknowledgement signal indicating whether thereception-side terminal has been able to receive the user data. Theprocess A comprises: a process of directly receiving the transmissionacknowledgement signal from the reception-side terminal and receivingthe transmission acknowledgement signal via the radio base station.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a mobile communication system 100according to the first embodiment.

FIG. 2 is a diagram illustrating the radio frame according to the firstembodiment.

FIG. 3 is a diagram illustrating the radio resource according to thefirst embodiment.

FIG. 4 is a diagram illustrating the application case according to thefirst embodiment.

FIG. 5 is a diagram for explaining the first retransmission controlexample according to the first embodiment.

FIG. 6 is a diagram for explaining the second retransmission controlexample according to the first embodiment.

FIG. 7 is a diagram for explaining the third retransmission controlexample according to the first embodiment.

FIG. 8 is a diagram for explaining the fourth retransmission controlexample according to the first embodiment.

FIG. 9 is a diagram for explaining the fifth retransmission controlexample according to the first embodiment.

FIG. 10 is a diagram for explaining the reception resource according tothe first embodiment.

FIG. 11 is a diagram for explaining the reception resource according tothe first embodiment.

FIG. 12 is a diagram illustrating the UE 10A (transmission-sideterminal) according to the first embodiment.

FIG. 13 is a diagram illustrating the UE 10B (reception-side terminal)according to the first embodiment.

FIG. 14 is a diagram illustrating the radio base station 310 accordingto the first embodiment.

FIG. 15 is a diagram illustrating the operation of the mobilecommunication system 100 according to the first embodiment.

FIG. 16 is a diagram illustrating the operation of the mobilecommunication system 100 according to the first embodiment.

FIG. 17 is a diagram illustrating the operation of the mobilecommunication system 100 according to the first embodiment.

FIG. 18 is a diagram illustrating the operation of the mobilecommunication system 100 according to the first embodiment.

FIG. 19 is a diagram illustrating the operation of the mobilecommunication system 100 according to the first embodiment.

FIG. 20 is a diagram illustrating the operation of the mobilecommunication system 100 according to the first embodiment.

FIG. 21 is a diagram illustrating the operation of the mobilecommunication system 100 according to the first modification.

FIG. 22 is a diagram illustrating the operation of the mobilecommunication system 100 according to the first modification.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a mobile communication system according to an embodiment ofthe present invention will be described with reference to theaccompanying drawings. Note that in the descriptions of the drawingbelow, identical or similar symbols are assigned to identical or similarportions.

It will be appreciated that the drawings are schematically shown and theratio and the like of each dimension are different from the real ones.Accordingly, specific dimensions should be determined in considerationof the explanation below. Of course, among the drawings, the dimensionalrelationship and the ratio may be different.

SUMMARY OF THE EMBODIMENT

A mobile communication system according to the embodiment directlyperforms user-data communication between a plurality of radio terminalswithout involvement of a radio base station. The user-data communicationdirectly performed between the plurality of radio terminals is performedby using a part of radio resources assigned to the mobile communicationsystem. The plurality of radio terminals comprise: a transmission-sideterminal that transmits the user data; and a reception-side terminalthat receives the user data. The reception-side terminal comprises: areception-side transmission unit that transmits a transmissionacknowledgement signal indicating whether the user data transmitted fromthe transmission-side terminal has been able to receive or not. Thereception-side transmission unit transmits the transmissionacknowledgement signal to the radio base station.

In the embodiment, the reception-side terminal transmits thetransmission acknowledgement signal to the radio base station.Consequently, it is possible to appropriately perform the retransmissioncontrol of the user data.

For example, when the radio base station receives the user datacommunicated between a plurality of radio terminals, since thetransmission acknowledgement signal is transmitted to the radio basestation, the radio base station is able to perform the retransmissioncontrol of the user data. Meanwhile, when the transmission-side terminalperforms the retransmission control of the user data, it is sufficientif the radio base station transmits the transmission acknowledgementsignal to the transmission-side terminal.

Furthermore, the communication directly performed between the pluralityof radio terminals without involvement of the radio base station may becalled D2D communication. The D2D communication is performed by using apart (a D2D radio resource) of the radio resources assigned to themobile communication system. As the D2D radio resource, for example, apart of uplink radio resources is used.

Furthermore, a radio resource used in the user-data communication in theD2D communication may be assigned by a radio base station or a radioterminal (a transmission-side terminal or a reception-side terminal).

First Embodiment Mobile Communication System

Hereinafter, a mobile communication system according to a firstembodiment will be described. FIG. 1 is a diagram illustrating a mobilecommunication system 100 according to the first embodiment.

As illustrated in FIG. 1, the mobile communication system 100 includes aradio terminal 100 (hereinafter, UE 10) and a core network 50.Furthermore, the mobile communication system 100 includes a firstcommunication system and a second communication system.

The first communication system, for example, is a communication systemcorresponding to LTE (Long Term Evolution). The first communicationsystem, for example, includes a base station 110A (hereinafter, MeNB110A), a home base station 110B (hereinafter, HeNB 110B), a home basestation gateway 120B (hereinafter, HeNB-GW 120B), and MME 130.

A radio access network (E-UTRAN; Evolved Universal Terrestrial RadioAccess Network) corresponding to the first communication system isconfigured from the MeNB 110A, the HeNB 110B, and the HeNB-GW 120B.

The second communication system, for example, is a communication systemcorresponding to UMTS (Universal Mobile Telecommunication System). Thesecond communication system includes a base station 210A (hereinafter,MNB 210A), a home base station 210B (hereinafter, HNB 210B), RNC 220A, ahome base station gateway 220B (hereinafter, HNB-GW 220B), and SGSN 230.

A radio access network (UTRAN; Universal Terrestrial Radio AccessNetwork) corresponding to the second communication system is configuredfrom the MNB 210A, the HNB 210B, the RNC 220A, and the HNB-GW 220B.

The UE 10 is a device (User Equipment) configured to communicate withthe second communication system or the first communication system. Forexample, the UE 10 has a function of performing radio communication withthe MeNB 110A and the HeNB 110B. Alternatively, the UE 10 has a functionof performing radio communication with the MNB 210A and the HNB 210B.

The MeNB 110A, which manages a general cell 111A, is a device (evolvedNodeB) configured to perform radio communication with the UE 10 beingpresent in the general cell 111A.

The HeNB 110B, which manages a specific cell 111B, is a device (Homeevolved NodeB) configured to perform radio communication with the UE 10being present in the specific cell 111B.

The HeNB-GW 120B, which is connected to the HeNB 110B, is a device (Homeevolved NodeB Gateway) configured to manage the HeNB 110B.

The MME 130, which is connected to the MeNB 110A, is a device (MobilityManagement Entity) configured to manage the mobility of the UE 10 havingset up a radio connection with the MeNB 110A. Furthermore, the MME 130,which is connected to the HeNB 110B via the HeNB-GW 120B, is a deviceconfigured to manage the mobility of the UE 10 having set up a radioconnection with the HeNB 110B.

The MNB 210A, which manages a general cell 211A, is a device (NodeB)configured to perform radio communication with the UE 10 being presentin the general cell 211A.

The HNB 210B, which manages a specific cell 211B, is a device (HomeNodeB) configured to perform radio communication with the UE 10 beingpresent in the specific cell 211B.

The RNC 220A, which is connected to the MNB 210A, is a device (RadioNetwork Controller) configured to set up a radio connection (RRCConnection) with the UE 10 being present in the general cell 211A.

The HNB-GW 220B, which is connected to the HNB 210B, is a device (HomeNodeB Gateway) configured to set up a radio connection (RRC Connection)with the UE 10 being present in the specific cell 211B.

The SGSN 230 is a device (Serving GPRS Support Node) configured toperform packet switching in a packet switching domain. The SGSN 230 isprovided in the core network 50. Although not illustrated in FIG. 1, adevice (MSC; Mobile Switching Center) configured to perform circuitswitching in a circuit switching domain may be provided in the corenetwork 50.

Furthermore, the general cell and the specific cell must be understoodas functions of performing radio communication with the UE 10. However,the general cell and the specific cell are also used as terms indicatingcoverage areas of cells. Also, cells such as the general cell and thespecific cell are identified based on the frequency used in the cell,the spreading code, the time slot, or the like.

Here, a coverage area of the general cell is larger than a coverage areaof the specific cell. The general cell, for example, includes a macrocell provided by a communication provider. The specific cell, forexample, includes a femto cell or a home cell provided by the thirdparty other than the communication provider. The specific cell mayinclude a CSG (Closed Subscriber Group) cell or a pico cell provided bythe communication provider.

Hereinafter, the first communication system will be mainly described.The following description may also be applied to the secondcommunication system.

In the first communication system, OFDMA (Orthogonal Frequency DivisionMultiple Access) scheme is used as a downlink multiple scheme, andSC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme isused as an uplink multiple scheme.

Furthermore, in the first communication system, an uplink channelincludes an uplink control channel (PUCCH; Physical Uplink ControlChannel), an uplink shared channel (PUSCH; Physical Uplink SharedChannel), and the like. Furthermore, a downlink channel includes adownlink control channel (PDCCH; Physical Downlink Control Channel), adownlink shared channel (PDSCH; Physical Downlink Shared Channel), andthe like.

The uplink control channel is a channel used to transfer a controlsignal. The control signal, for example, includes CQI (Channel QualityIndicator), PMI (Precoding Matrix Indicator), RI (Rank Indicator), SR(Scheduling Request), and ACK/NACK.

The CQI is a signal for notifying a recommended modulation method and anencoding rate to be used in downlink transmission. The PMI is a signalindicating a precoder matrix which is preferably used in downlinktransmission. The RI is a signal indicating the number of layers (thenumber of streams) to be used in downlink transmission. The SR is asignal for requesting the assignment of an uplink radio resource (aresource block which will be described later). The ACK/NACK is a signalindicating whether a signal transmitted via the downlink channel (forexample, the PDSCH) has been successfully received.

The uplink shared channel is a channel used to transfer a control signal(including the above-mentioned control signal) and/or a data signal. Forexample, the uplink radio resource may be assigned only to the datasignal, or assigned such that the data signal and the control signal aremultiplexed.

The downlink control channel is a channel used to transfer a controlsignal. The control signal, for example, includes Uplink SI (SchedulingInformation), Downlink SI (Scheduling Information), and a TPC bit.

The Uplink SI is a signal indicating the assignment of the uplink radioresource. The Downlink SI is a signal indicating the assignment of adownlink radio resource. The TPC bit is a signal for instructingincrease or decrease in the power of a signal transmitted via the uplinkchannel.

The downlink shared channel is a channel used to transfer a controlsignal and/or a data signal. For example, the downlink radio resourcemay be assigned only to the data signal, or assigned such that the datasignal and the control signal are multiplexed.

In addition, a control signal transmitted via the downlink sharedchannel includes TA (Timing Advance). The TA is transmission timingcorrection information between the UE 10 and the MeNB 110A, and ismeasured by the MeNB 110A based on an uplink signal transmitted from theUE 10.

Furthermore, a control signal transmitted via a channel, other than thedownlink control channel (the PDCCH) and the downlink shared channel(the PDSCH), includes ACK/NACK. The ACK/NACK is a signal indicatingwhether a signal transmitted via the uplink channel (for example, thePUSCH) has been successfully received.

The general cell and the specific cell broadcast broadcast informationvia a broadcast channel (BCCH; Broadcast Control Channel). The broadcastinformation, for example, is information such as MIB (Master InformationBlock) or SIB (System Information Block).

Specifically not illustrated in FIG. 1, the first communication systemmay include a relay node configured to relay data communication betweenthe MeNB 110A (or the HeNB 110B) and the UE 10. Similarly, the secondcommunication system may include a relay node configured to relay datacommunication with the MNB 210A (or the HNB 210B).

(Radio Frame)

Hereinafter, a radio frame in the first communication system will bedescribed. FIG. 2 is a diagram illustrating the radio frame in the firstcommunication system.

As illustrated in FIG. 2, one radio frame is configured by 10 subframesand one subframe is configured by two slots. One slot has a time lengthof 0.5 msec, one subframe has a time length of 1 msec, and one radioframe has a time length of 10 msec.

In addition, one slot is configured by a plurality of OFDM symbols (forexample, six OFDM symbols or seven OFDM symbols) in the downwarddirection. In the same manner, one slot is configured by a plurality ofSC-FDMA symbols (for example, six SC-FDMA symbols or seven SC-FDMAsymbols) in the upward direction.

(Radio Resource)

Hereinafter, a radio resource in the first communication system will bedescribed. FIG. 3 is a diagram illustrating the radio resource in thefirst communication system.

As illustrated in FIG. 3, a radio resource is defined by a frequencyaxis and a time axis. A frequency is configured by a plurality ofsubcarriers, and a predetermined number of subcarriers (12 subcarriers)are collectively called a resource block (RB). A time has a unit, suchas the OFDM symbol (or the SC-FDMA symbol), the slot, the subframe, orthe radio frame, as described above.

Here, the radio resource is assignable to each one resource block.Furthermore, on the frequency axis and the time axis, it is possible todivide the radio resources to assign the same to a plurality of users(for example, user #1 to user #5).

Furthermore, the radio resource is assigned by the MeNB 110A. The MeNB110A assigns the radio resource to each UE 10 based on the CQI, the PMI,the RI, and the like.

(Application Case)

Hereinafter, the application case according to the first embodiment willbe described. FIG. 4 is a diagram for explaining the application caseaccording to the first embodiment. In FIG. 4, as the UE 10, UE 10A andUE 10B are illustrated. It is preferable that a radio base station 310is the MeNB 110A or the HeNB 110B. However, the radio base station 310may be the MNB 210A or the HNB 210B. Alternatively, the radio basestation 310 may be a relay node. A network device 330 is a deviceprovided in the core network 50. The network device 330 may be the MME130 or the SGSN 230.

As illustrated in FIG. 4, the communication of user data (data ofUser-Plane) is directly performed between a plurality of radio terminalswithout involvement of a radio base station (hereinafter, D2Dcommunication). Meanwhile, communication of control data (data ofC-Plane) is performed via the radio base station, similarly to that in aconventional mobile communication system.

Furthermore, the D2D communication is performed by using a part(hereinafter, a D2D radio resource) of radio resources assigned to themobile communication system. As the D2D radio resource, for example, apart of uplink radio resources is used. A radio resource used in theuser-data communication in the D2D communication may be assigned by aradio base station or a radio terminal (a transmission-side terminal ora reception-side terminal).

For example, it is preferable that the D2D radio resource is broadcastedfrom each cell managed by the radio base station. The D2D radioresource, for example, is included in MIB (Master Information Block) orSIB (System Information Block).

First Retransmission Control Example

Hereinafter, a first retransmission control example according to thefirst embodiment will be described. FIG. 5 is a diagram for explainingthe first retransmission control example according to the firstembodiment. In FIG. 5, as the UE 10, the UE 10A and the UE 10B areillustrated. The UE 10A is an example of a transmission-side terminaland the UE 10B is an example of a reception-side terminal.

As illustrated in FIG. 5, the UE 10B transmits, to the radio basestation 310, a transmission acknowledgement signal (ACK/NACK) indicatingwhether user data transmitted from the UE 10A has been able to bereceived. In response to the transmission acknowledgement signalreceived from the UE 10B, the radio base station 310 transmits thetransmission acknowledgement signal to the UE 10A. For example, theradio base station 310 may relay the transmission acknowledgementsignal, which is received from the UE 10B, to the UE 10A. Alternatively,the radio base station 310 may transmit the transmission acknowledgementsignal to the UE 10A, together with a signal for assigning a radioresource used in the user-data communication in the D2D communication tothe UE 10A.

When the transmission acknowledgement signal is NACK indicating that itis not possible to receive the user data, the UE 10A retransmits theuser data to the UE 10B.

It should be noted that, as described above, the first retransmissioncontrol example is the case in which the UE 10A (the transmission-sideterminal) performs retransmission control.

Second Retransmission Control Example

Hereinafter, a second retransmission control example according to thefirst embodiment will be described. FIG. 6 is a diagram for explainingthe second retransmission control example according to the firstembodiment. In FIG. 6, as the UE 10, the UE 10A and the UE 10B areillustrated. The UE 10A is an example of a transmission-side terminaland the UE 10B is an example of a reception-side terminal.

As illustrated in FIG. 6, the UE 10B transmits, to the UE 10A, atransmission acknowledgement signal (ACK/NACK) indicating whether userdata transmitted from the UE 10A has been able to be received. When thetransmission acknowledgement signal is NACK indicating that it is notpossible to receive the user data, the UE 10A retransmits the user datato the UE 10B.

It should be noted that, as described above, the second retransmissioncontrol example is the case in which the UE 10A (the transmission-sideterminal) performs retransmission control.

Third Retransmission Control Example

Hereinafter, a third retransmission control example according to thefirst embodiment will be described. FIG. 7 is a diagram for explainingthe third retransmission control example according to the firstembodiment. In FIG. 7, as the UE 10, the UE 10A and the UE 10B areillustrated. The UE 10A is an example of a transmission-side terminaland the UE 10B is an example of a reception-side terminal.

As illustrated in FIG. 7, the UE 10B transmits, to the UE 10A and theradio base station 310, a transmission acknowledgement signal (ACK/NACK)indicating whether user data transmitted from the UE 10A has been ableto be received. In response to the transmission acknowledgement signalreceived from the UE 10B, the radio base station 310 transmits thetransmission acknowledgement signal to the UE 10A. For example, theradio base station 310 may relay the transmission acknowledgementsignal, which is received from the UE 10B, to the UE 10A. Alternatively,the radio base station 310 may transmit the transmission acknowledgementsignal to the UE 10A, together with a signal for assigning a radioresource used in the user-data communication in the D2D communication tothe UE 10A. The UE 10A performs retransmission control of the user dataon the basis of the transmission acknowledgement signal received fromthe UE 10B and the transmission acknowledgement signal received via theradio base station 310.

For example, when one of the transmission acknowledgement signalsreceived from the UE 10B and the radio base station 310 is NACK, the UE10A retransmits the user data to the UE 10B. Alternatively, when acommunication state of the user data directly performed between the UE10A and the UE 10B is good, the UE 10A may refer to the transmissionacknowledgement signal received from the UE 10B without referring to thetransmission acknowledgement signal received via the radio base station310. In such a case, it is preferable that the UE 10A notifies the radiobase station 310 of whether to refer to the transmission acknowledgementsignal received via the radio base station 310. When the radio basestation 310 is notified of the fact that the transmissionacknowledgement signal is not referred to, it is preferable that theradio base station 310 omits the transmission of the transmissionacknowledgement signal to the UE 10A.

Alternatively, when the communication state of the user data directlyperformed between the UE 10A and the UE 10B is good, the UE 10B may omitthe transmission of the transmission acknowledgement signal to the radiobase station 310. In other words, when the communication state of theuser data directly performed between the UE 10A and the UE 10B is poor,the UE 10B transmits the transmission acknowledgement signal to theradio base station 310.

Furthermore, the case in which the communication state is good includesthe case in which transmission power used for the user-datacommunication is below a threshold value, or the case in which amodulation coding scheme used for the user-data communication exceeds athreshold value. Alternatively, the case in which the communicationstate is good may include the case in which a block error rate is belowa threshold value, the case in which a packet error rate is below athreshold value, the case in which QoS is satisfied, the case in whichCQI exceeds a threshold value, and the case in which a processing loadof the UE 10A is below a threshold value.

Furthermore, the case in which the communication state is poor includesthe case in which the transmission power used for the user-datacommunication exceeds a threshold value, or the case in which themodulation coding scheme used for the user-data communication is below athreshold value. Alternatively, the case in which the communicationstate is poor may include the case in which the block error rate exceedsa threshold value, the case in which the packet error rate exceeds athreshold value, the case in which the QoS is not satisfied, the case inwhich the CQI is below a threshold value, and the case in which theprocessing load of the UE 10A exceeds a threshold value.

It should be noted that, as described above, the third retransmissioncontrol example is the case in which the UE 10A (the transmission-sideterminal) performs retransmission control.

Fourth Retransmission Control Example

Hereinafter, a fourth retransmission control example according to thefirst embodiment will be described. FIG. 8 is a diagram for explainingthe fourth retransmission control example according to the firstembodiment. In FIG. 8, as the UE 10, the UE 10A and the UE 10B areillustrated. The UE 10A is an example of a transmission-side terminaland the UE 10B is an example of a reception-side terminal.

As illustrated in FIG. 8, the UE 10B transmits, to the radio basestation 310, a transmission acknowledgement signal (ACK/NACK) indicatingwhether user data transmitted from the UE 10A has been able to bereceived. The radio base station 310 assigns a radio resource, which isassigned to the user-data communication directly performed between theUE 10A and the UE 10B, as a reception resource for receiving the userdata. In this way, the radio base station 310 is able to receive theuser data transmitted from the UE 10A to the UE 10B. When thetransmission acknowledgement signal is NACK indicating that it is notpossible to receive the user data, the radio base station 310retransmits the user data to the UE 10B.

It should be noted that, as described above, the fourth retransmissioncontrol example is the case in which the radio base station 310 performsretransmission control.

Fifth Retransmission Control Example

Hereinafter, a fifth retransmission control example according to thefirst embodiment will be described. FIG. 9 is a diagram for explainingthe fifth retransmission control example according to the firstembodiment. In FIG. 9, as the UE 10, the UE 10A and the UE 10B areillustrated. The UE 10A is an example of a transmission-side terminaland the UE 10B is an example of a reception-side terminal.

As illustrated in FIG. 9, the UE 10B transmits, to the UE 10A, atransmission acknowledgement signal (ACK/NACK) indicating whether userdata transmitted from the UE 10A has been able to be received. When thetransmission acknowledgement signal is NACK indicating that it is notpossible to receive the user data, the UE 10A transmits a retransmissionrequest of the user data to the radio base station 310. The radio basestation 310 assigns a radio resource, which is assigned to the user-datacommunication directly performed between the UE 10A and the UE 10B, as areception resource for receiving the user data. In this way, the radiobase station 310 is able to receive the user data transmitted from theUE 10A to the UE 10B. When the retransmission request is received, theradio base station 310 retransmits the user data to the UE 10B.

It should be noted that, as described above, the fifth retransmissioncontrol example is the case in which the radio base station 310 performsretransmission control.

(Reception Resource)

Hereinafter, the reception resource according to the first embodimentwill be described. FIG. 10 and FIG. 11 are diagrams for explaining thereception resource according to the first embodiment.

As illustrated in FIG. 10, when the radio resource assigned to the D2Dcommunication is not assigned as the reception resource for receivingthe user data transmitted from the UE 10A to the UE 10B, the radio basestation 310 is not able to receive the user data transmitted from the UE10A to the UE 10B. In such a case, the radio base station 310 is able toreceive uplink user data from another UE 10 by using the radio resourceassigned to the D2D communication.

As illustrated in FIG. 11, when the radio resource assigned to the D2Dcommunication is assigned as the reception resource for receiving theuser data transmitted from the UE 10A to the UE 10B, the radio basestation 310 is able to receive the user data transmitted from the UE 10Ato the UE 10B. In such a case, the radio base station 310 is not able toreceive uplink user data from another UE 10 by using the radio resourceassigned to the D2D communication.

(Transmission-Side Terminal)

Hereinafter, the transmission-side terminal according to the firstembodiment will be described. Furthermore, as the transmission-sideterminal, the UE 10A is illustrated. FIG. 12 is a block diagramillustrating the UE 10A according to the first embodiment.

As illustrated in FIG. 12, the UE 10A includes a reception unit 13A, atransmission unit 14A, and a control unit 15A.

The reception unit 13A receives data from the radio base station 310 incellular communication. The reception unit 13A receives data from the UE10B in the D2D communication. For example, in the D2D communication, thereception unit 13A may receive, from the UE 10B, a transmissionacknowledgement signal (ACK/NACK) indicating whether it is possible toreceive user data. As described above, the reception unit 13A mayreceive the transmission acknowledgement signal via the radio basestation 310 in the D2D communication.

The transmission unit 14A transmits data to the radio base station 310in the cellular communication. The transmission unit 14A transmits datato the UE 10B in the D2D communication. For example, the transmissionunit 14A transmits user data to the UE 10B in the D2D communication.Furthermore, the transmission unit 14A retransmits the user data to theUE 10B in response to an instruction output from the control unit 15A.

Furthermore, as described in the aforementioned fifth retransmissioncontrol example, when the transmission acknowledgement signal is NACKindicating that it is not possible to receive the user data, thetransmission unit 14A may transmit a retransmission request of the userdata to the radio base station 310.

The control unit 15A controls the UE 10A. Specifically, on the basis ofthe transmission acknowledgement signal received from the UE 10B and thetransmission acknowledgement signal received via the radio base station310, the control unit 15A determines whether to retransmit the user datato the UE 10B. That is, the control unit 15A performs retransmissioncontrol of the user data. When determining to retransmit the user data,the control unit 15A instructs the transmission unit 14A to retransmitthe user data.

For example, when one of the transmission acknowledgement signalsreceived from the UE 10B and the radio base station 310 is NACK, thecontrol unit 15A determines to retransmit the user data to the UE 10B.Alternatively, when the communication state of the user data directlyperformed between the UE 10A and the UE 10B is good, the control unit15A may refer to the transmission acknowledgement signal received fromthe UE 10B without referring to the transmission acknowledgement signalreceived via the radio base station 310. In such a case, it ispreferable that the control unit 15A notifies the radio base station 310of whether to refer to the transmission acknowledgement signal receivedvia the radio base station 310.

(Reception-Side Terminal)

Hereinafter, the reception-side terminal according to the firstembodiment will be described. Furthermore, as the reception-sideterminal, the UE 10B is illustrated. FIG. 13 is a block diagramillustrating the UE 10B according to the first embodiment.

As illustrated in FIG. 13, the UE 10B includes a reception unit 13B, atransmission unit 14B, and a control unit 15B.

The reception unit 13B receives data from the radio base station 310 inthe cellular communication. The reception unit 13B receives data fromthe UE 10A in the D2D communication. For example, in the D2Dcommunication, the reception unit 13B receives user data (initialtransmission) transmitted from the UE 10A. Furthermore, the transmissionunit 14A receives user data (retransmission) retransmitted from the UE10A.

The transmission unit 14B transmits data to the radio base station 310in the cellular communication. The transmission unit 14B transmits datato the UE 10A in the D2D communication. For example, the transmissionunit 14B may transmit, to the UE 10A, a transmission acknowledgementsignal (ACK/NACK) indicating whether it is possible to receive userdata. In the first embodiment, the transmission unit 14B may transmitthe transmission acknowledgement signal (ACK/NACK) to the radio basestation 310 in the D2D communication.

The control unit 15B controls the UE 10B. For example, the control unit15B determines whether it is possible to receive the user datatransmitted from the UE 10A and instructs the transmission unit 14B totransmit the transmission acknowledgement signal. Specifically, when itis possible to receive the user data, the control unit 15B instructs thetransmission unit 14B to transmit ACK. Meanwhile, when it is notpossible to receive the user data, the control unit 15B instructs thetransmission unit 14B to transmit NACK.

When the communication state of the user data directly performed betweenthe UE 10A and the UE 10B is good, the control unit 15B may omit thetransmission of the transmission acknowledgement signal to the radiobase station 310. In other words, when the communication state of theuser data directly performed between the UE 10A and the UE 10B is poor,the control unit 15B instructs the transmission unit 14B to transmit thetransmission acknowledgement signal to the radio base station 310. Thatis, when the communication state of the user data is poor, thetransmission unit 14B transmits the transmission acknowledgement signalto the radio base station 310.

(Radio Base Station)

Hereinafter, the radio base station according to the first embodimentwill be described. FIG. 14 is a block diagram illustrating the radiobase station 310 according to the first embodiment.

As illustrated in FIG. 14, the radio base station 310 includes areception unit 313, a transmission unit 314, and a control unit 315.

The reception unit 313 receives data from the UE 10. For example, in theD2D communication, the reception unit 313 receives, from the UE 10B, atransmission acknowledgement signal (ACK/NACK) indicating whether it ispossible to receive user data. Furthermore, the reception unit 313 mayreceive the user data transmitted from the UE 10A to the UE 10B.

The transmission unit 314 transmits data to the UE 10. For example, inthe D2D communication, in response to the transmission acknowledgementsignal (ACK/NACK) received from the UE 10B, the transmission unit 314transmits the transmission acknowledgement signal to the UE 10A. Forexample, the transmission unit 314 may relay the transmissionacknowledgement signal, which is received from the UE 10B, to the UE10A. Alternatively, the transmission unit 314 may transmit thetransmission acknowledgement signal to the UE 10A, together with asignal for assigning the radio resource used in the user-datacommunication to the UE 10A in the D2D communication. Furthermore, inthe D2D communication, when NACK indicating that it is not possible toreceive the user data is received from the UE 10B, the transmission unit314 retransmits the user data to the UE 10B. Alternatively, in the D2Dcommunication, when a retransmission request is received from the UE10A, the transmission unit 314 retransmits the user data to the UE 10B.

The control unit 315 controls the radio base station 310. Specifically,the control unit 315 assigns uplink and downlink radio resources to theUE 10. Furthermore, the control unit 315 may assign the radio resource,which is assigned to the user-data communication directly performedbetween the UE 10A and the UE 10B, as the reception resource forreceiving the user data.

In the first embodiment, on the basis of a reception state of the userdata received by using the reception resource in the D2D communication,the control unit 315 may determine whether to transmit the transmissionacknowledgement signal. For example, when the reception state of theuser data is good, the control unit 315 instructs the transmission unit314 to omit the transmission of the transmission acknowledgement signal.In other words, when the reception state of the user data is poor, thecontrol unit 315 instructs the transmission unit 314 to transmit thetransmission acknowledgement signal.

In the first embodiment, on the basis of the transmissionacknowledgement signal received from the UE 10B in the D2Dcommunication, the control unit 315 determines whether to retransmit theuser data to the UE 10B. That is, the control unit 315 performsretransmission control of the user data. When determining to retransmitthe user data, the control unit 315 instructs the transmission unit 14Ato retransmit the user data received by using the reception resource.

(Operation of Mobile Communication System)

Hereinafter, the operation of the mobile communication system accordingto the first embodiment will be described. FIG. 15 to FIG. 20 arediagrams illustrating the operation of a mobile communication system 100according to the first embodiment.

Firstly, the first retransmission control example illustrated in FIG. 5will be described with reference to FIG. 15. In the case illustrated inFIG. 15, a radio resource used in the user-data communication in the D2Dcommunication is assigned by the radio base station 310 andretransmission control of the user data is performed by the UE 10A.

As illustrated in FIG. 15, in step 10, the radio base station 310assigns, to the UE 10A and the UE 10B, the radio resource used in theuser-data communication in the D2D communication from among D2D radioresources. The radio base station 310 notifies the UE 10A and the UE 10Bof the assigned radio resource.

In step 20, the UE 10A transmits user data (initial transmission) to theUE 10B by using the radio resource assigned in step 10. Similarly, theUE 10B receives the user data (initial transmission) from the UE 10A byusing the radio resource assigned in step 10.

In step 30, the UE 10B transmits, to the radio base station 310, atransmission acknowledgement signal indicating whether the user data(initial transmission) has been able to be received. Hereinafter, thefollowing description will be given on the assumption that NACK istransmitted as the transmission acknowledgement signal.

In step 40, in response to the transmission acknowledgement signalreceived from the UE 10B, the radio base station 310 transmits thetransmission acknowledgement signal to the UE 10A. Furthermore, theradio base station 310 may relay the transmission acknowledgementsignal, which is received from the UE 10B, to the UE 10A. In addition,the radio base station 310 may transmit the transmission acknowledgementsignal to the UE 10A, together with a signal for assigning the radioresource used in the user-data communication in the D2D communication tothe UE 10A (for example, step 50).

In step 50, the radio base station 310 assigns, to the UE 10A and the UE10B, the radio resource used in the user-data communication in the D2Dcommunication from among the D2D radio resources. The radio base station310 notifies the UE 10A and the UE 10B of the assigned radio resource.

In step 60, the UE 10A transmits user data (retransmission) to the UE10B by using the radio resource assigned in step 50. Similarly, the UE10B receives the user data (retransmission) from the UE 10A by using theradio resource assigned in step 50.

Secondly, the second retransmission control example illustrated in FIG.6 will be described with reference to FIG. 16. In the case illustratedin FIG. 16, the radio resource used in the user-data communication inthe D2D communication is assigned by the UE 10 (the UE 10A or the UE10B) and retransmission control of the user data is performed by the UE10A.

As illustrated in FIG. 16, in step 10A, the UE 10 (the UE 10A or the UE10B) autonomously assigns the radio resource used in the user-datacommunication in the D2D communication from among D2D radio resources.The UE 10 notifies another UE 10 of the assigned radio resource.

In step 20A, the UE 10A transmits user data (initial transmission) tothe UE 10B by using the radio resource assigned in step 10A. Similarly,the UE 10B receives the user data (initial transmission) from the UE 10Aby using the radio resource assigned in step 10A.

In step 30A, the UE 10B transmits, to the radio base station 310, atransmission acknowledgement signal indicating whether the user data(initial transmission) has been able to be received. Hereinafter, thefollowing description will be given on the assumption that NACK istransmitted as the transmission acknowledgement signal.

In step 40A, in response to the transmission acknowledgement signalreceived from the UE 10B, the radio base station 310 transmits thetransmission acknowledgement signal to the UE 10A. Furthermore, theradio base station 310 may relay the transmission acknowledgementsignal, which is received from the UE 10B, to the UE 10A. In addition,the radio base station 310 may transmit the transmission acknowledgementsignal to the UE 10A, together with a signal for assigning the radioresource used in the user-data communication in the D2D communication tothe UE 10A (for example, step 50A).

In step 50A, the UE 10 (the UE 10A or the UE 10B) autonomously assignsthe radio resource used in the user-data communication in the D2Dcommunication from among the D2D radio resources. The UE 10 notifiesanother UE 10 of the assigned radio resource.

In step 60A, the UE 10A transmits user data (retransmission) to the UE10B by using the radio resource assigned in step 50A. Similarly, the UE10B receives the user data (retransmission) from the UE 10A by using theradio resource assigned in step 50A.

Thirdly, the third retransmission control example illustrated in FIG. 7will be described with reference to FIG. 17. In the case illustrated inFIG. 17, the radio resource used in the user-data communication in theD2D communication is assigned by the UE 10 (the UE 10A or the UE 10B)and retransmission control of the user data is performed by the UE 10A.

As illustrated in FIG. 17, in step 10B, the UE 10 (the UE 10A or the UE10B) autonomously assigns the radio resource used in the user-datacommunication in the D2D communication from among D2D radio resources.The UE 10 notifies another UE 10 of the assigned radio resource.

In step 20B, the UE 10A transmits user data (initial transmission) tothe UE 10B by using the radio resource assigned in step 10B. Similarly,the UE 10B receives the user data (initial transmission) from the UE 10Aby using the radio resource assigned in step 10B.

In step 30B, the UE 10B transmits, to the UE 10A, a transmissionacknowledgement signal indicating whether the user data (initialtransmission) has been able to be received. Hereinafter, the followingdescription will be given on the assumption that NACK is transmitted asthe transmission acknowledgement signal.

In step 50B, the UE 10 (the UE 10A or the UE 10B) autonomously assignsthe radio resource used in the user-data communication in the D2Dcommunication from among the D2D radio resources. The UE 10 notifiesanother UE 10 of the assigned radio resource.

In step 60B, the UE 10A transmits user data (retransmission) to the UE10B by using the radio resource assigned in step 50B. Similarly, the UE10B receives the user data (retransmission) from the UE 10A by using theradio resource assigned in step 50B.

Fourthly, the fourth retransmission control example illustrated in FIG.8 will be described with reference to FIG. 18. In the case illustratedin FIG. 18, the radio resource used in the user-data communication inthe D2D communication is assigned by the radio base station 310 andretransmission control of the user data is performed by the radio basestation 310. It should be noted that, in the case illustrated in FIG.18, the radio base station 310 assigns the radio resource, which isassigned to the user-data communication directly performed between theUE 10A and the UE 10B, as the reception resource for receiving the userdata.

As illustrated in FIG. 18, in step 10C, the radio base station 310assigns, to the UE 10A and the UE 10B, the radio resource used in theuser-data communication in the D2D communication from among D2D radioresources. The radio base station 310 notifies the UE 10A and the UE 10Bof the assigned radio resource.

In step 20C, the UE 10A transmits, to the UE 10B and the radio basestation 310, user data (initial transmission) by using the radioresource assigned in step 10C. Similarly, the UE 10B and the radio basestation 310 receive the user data (initial transmission) from the UE 10Aby using the radio resource assigned in step 10C.

In step 30C, the UE 10B transmits, to the radio base station 310, atransmission acknowledgement signal indicating whether the user data(initial transmission) has been able to be received. Hereinafter, thefollowing description will be given on the assumption that NACK istransmitted as the transmission acknowledgement signal.

In step 50C, the radio base station 310 assigns, to the UE 10B, theradio resource used in the user-data communication in the D2Dcommunication from among the D2D radio resources. The radio base station310 notifies the UE 10B of the assigned radio resource.

In step 60C, the radio base station 310 transmits user data(retransmission) to the UE 10B by using the radio resource assigned instep 50C. Similarly, the UE 10B receives the user data (retransmission)from the radio base station 310 by using the radio resource assigned instep 50C.

Fifthly, the fifth retransmission control example illustrated in FIG. 9will be described with reference to FIG. 19. In the case illustrated inFIG. 19, the radio resource used in the user-data communication in theD2D communication is assigned by the radio base station 310 andretransmission control of the user data is performed by the radio basestation 310. It should be noted that, in the case illustrated in FIG.19, the radio base station 310 assigns the radio resource, which isassigned to the user-data communication directly performed between theUE 10A and the UE 10B, as the reception resource for receiving the userdata.

As illustrated in FIG. 19, in step 10D, the radio base station 310assigns, to the UE 10A and the UE 10B, the radio resource used in theuser-data communication in the D2D communication from among D2D radioresources. The radio base station 310 notifies the UE 10A and the UE 10Bof the assigned radio resource.

In step 20D, the UE 10A transmits, to the UE 10B and the radio basestation 310, user data (initial transmission) by using the radioresource assigned in step 10D. Similarly, the UE 10B and the radio basestation 310 receive the user data (initial transmission) from the UE 10Aby using the radio resource assigned in step 10D.

In step 30D, the UE 10B transmits, to the UE 10A, a transmissionacknowledgement signal indicating whether the user data (initialtransmission) has been able to be received. Hereinafter, the followingdescription will be given on the assumption that NACK is transmitted asthe transmission acknowledgement signal.

In step 40D, the UE 10A transmits a retransmission request of the userdata to the radio base station 310.

In step 50D, the radio base station 310 assigns, to the UE 10B, theradio resource used in the user-data communication in the D2Dcommunication from among the D2D radio resources. The radio base station310 notifies the UE 10B of the assigned radio resource.

In step 60D, the radio base station 310 transmits user data(retransmission) to the UE 10B by using the radio resource assigned instep 50D. Similarly, the UE 10B receives the user data (retransmission)from the radio base station 310 by using the radio resource assigned instep 50D.

Sixthly, a different retransmission control for performing aretransmission control of user data by the radio base station 310 willbe described with reference to FIG. 20. In the case illustrated in FIG.20, the radio resource used in the user-data communication in the D2Dcommunication is assigned by the UE 10 (here, the UE 10A) andretransmission control of the user data is performed by the radio basestation 310. In addition, the radio resource used in the user-datacommunication in the D2D communication may be assigned by the UE 10B. Itshould be noted that, in the case illustrated in FIG. 20, the radio basestation 310 assigns the radio resource, which is assigned to theuser-data communication performed between the UE 10A and the UE 10B, asthe reception resource for receiving the user data.

As illustrated in FIG. 20, in step 10E, the UE 10A autonomously assignsthe radio resource used in the user-data communication in the D2Dcommunication from among D2D radio resources. The UE 10 notifies anotherUE 10 of the assigned radio resource.

In step 15E, the UE 10A notifies the radio base station 310 of the radioresource assigned in step 10E.

In step 20E, the UE 10A transmits, to the UE 10B and the radio basestation 310, user data (initial transmission) by using the radioresource assigned in step 10E. Similarly, the UE 10B and the radio basestation 310 receive the user data (initial transmission) from the UE 10Aby using the radio resource assigned in step 10E.

In step 30E, the UE 10B transmits, to the UE 10A, a transmissionacknowledgement signal indicating whether the user data (initialtransmission) has been able to be received. Hereinafter, the followingdescription will be given on the assumption that NACK is transmitted asthe transmission acknowledgement signal.

In step 50E, the UE 10A autonomously assigns the radio resource used inthe user-data communication in the D2D communication from among the D2Dradio resources. The UE 10A notifies the UE 10B of the assigned radioresource.

In step 55E, the UE 10A notifies the radio base station 310 of the radioresource assigned in step 50E, together with a retransmission request ofthe user data.

In step 60E, the radio base station 310 transmits user data(retransmission) by using the radio resource assigned in step 50E.Similarly, the UE 10B receives the user data (retransmission) from theradio base station 310 by using the radio resource assigned in step 50E.

(Operation and Effect)

In the first embodiment, the UE 10B (the reception-side terminal)transmits the transmission acknowledgement signal to the radio basestation 310. Accordingly, it is possible to appropriately perform theretransmission control of the user data.

For example, when the radio base station 310 receives the user datacommunicated between the plurality of UEs 10, since the transmissionacknowledgement signal is transmitted to the radio base station 310, theradio base station 310 is able to perform the retransmission control ofthe user data. Meanwhile, when the UE 10A (the transmission-sideterminal) performs the retransmission control of the user data, it issufficient if the radio base station 310 transmits the transmissionacknowledgement signal to the transmission-side terminal.

In the first embodiment, the radio base station 310 assigns thereception resource for receiving the user data directly communicatedbetween the plurality of UEs 10. That is, the radio base station 310 isable to receive the user data communicated between the plurality of UEs10. Accordingly, for example, the transmission acknowledgement signal istransmitted to the radio base station 310, so that the radio basestation 310 is able to perform the retransmission control of the userdata.

[First Modification]

Hereinafter, a first modification of the first embodiment will beexplained. Mainly the differences from the first embodiment will bedescribed, below.

In the first modification, the radio base station 310 (the control unit315) determines whether to assign the radio resource, which is assignedto the user-data communication directly performed between the UE 10A andthe UE 10B, as the reception resource for receiving the user data.Specifically, when the communication state of the user data directlyperformed between the UE 10A and the UE 10B is poor, the radio basestation 310 (the control unit 315) assigns the radio resource assignedto the user-data communication as the reception resource for receivingthe user data. Similarly, when the communication state of the user datadirectly performed between the UE 10A and the UE 10B is poor, the radiobase station 310 (the transmission unit 314 and the control unit 315)performs the retransmission control of the user data.

Furthermore, it is preferable that the radio base station 310 (thereception unit 313) receives the D2D control signal for controllingdirect user-data communication from the UE 10 (the UE 10A or the UE10B). In such a case, on the basis of the D2D control signal, the radiobase station 310 (the control unit 315) determines whether to assign thereception resource for receiving the user data directly communicatedbetween the UE 10A and the UE 10B. Similarly, on the basis of the D2Dcontrol signal, the radio base station 310 (the transmission unit 314and the control unit 315) determines whether to perform theretransmission control of the user data.

In the first modification, the D2D control signal may be transmittedfrom the UE 10A (the transmission unit 14A) to the radio base station310, or may be transmitted from the UE 10B (the transmission unit 14B)to the radio base station 310.

It is preferable that the D2D control signal indicates at least one of:a transmission acknowledgement signal indicating whether thereception-side terminal has been able to receive user data transmittedfrom the UE 10A; transmission power used in the user-data communication;and a modulation coding scheme used in the user-data communication. Itshould be noted that, in such a case, the radio base station 310 (thecontrol unit 315) determines, on the basis of the D2D control signal,whether the communication state of the user data directly performedbetween the UE 10A and the UE 10B is poor.

Alternatively, it is preferable that the D2D control signal indicates atleast one of: a signal for requesting the retransmission control of theuser data; a signal indicating that the transmission power used in theuser-data communication has exceeded a threshold value; and a signalindicating that the modulation coding scheme used in the user-datacommunication has been below a threshold value. It should be noted that,in such a case, the radio base station 310 (the control unit 315)determines whether the communication state of the user data directlyperformed between the UE 10A and the UE 10B is poor.

Alternatively, the D2D control signal may indicate at least one of: asignal indicating that the transmission power used in the user-datacommunication has been below a threshold value; and a signal indicatingthat the modulation coding scheme used in the user-data communicationhas exceeded a threshold value. In such a case, the UE 10 (the controlunit 15A or the control unit 15B) is able to determine whether thecommunication state of the user data directly performed between the UE10A and the UE 10B is good.

Alternatively, the D2D control signal may indicate a signal forinstructing the radio base station 310 whether to assign the radioresource, which was assigned to the user-data communication performedbetween the UE 10A and the UE 10B, as the reception resource forreceiving the user data.

(Operation of Mobile Communication System)

Hereinafter, the operation of a mobile communication system according tothe first modification will be described. FIG. 21 and FIG. 22 arediagrams illustrating the operation of a mobile communication system 100according to the first modification.

Firstly, with reference to FIG. 21, a description will be provided forthe case in which the radio base station 310 determines whether thecommunication state of user data in the D2D communication is poor.

As illustrated in FIG. 21, in step 110, the radio base station 310assigns, to the UE 10A and the UE 10B, a radio resource used in theuser-data communication in the D2D communication from among the D2Dradio resources. The radio base station 310 notifies the UE 10A and theUE 10B of the assigned radio resource.

In step 120, the UE 10A transmits the D2D control signal to the radiobase station 310. Furthermore, it is preferable that the D2D controlsignal indicates at least one of: a transmission acknowledgement signalindicating whether the reception-side terminal has been able to receiveuser data transmitted from the UE 10A; transmission power used in theuser-data communication; and a modulation coding scheme used in theuser-data communication.

In step 130, the radio base station 310 determines whether to receiveuser data directly communicated between the UE 10A and the UE 10B (toperform simultaneous reception of the user data). Specifically, when thecommunication state of the user data directly performed between the UE10A and the UE 10B is poor, the radio base station 310 determines toperform the simultaneous reception of the user data.

Specifically, on the basis of the D2D control signal, the radio basestation 310 determines whether the communication state of the user datais poor. For example, when the transmission acknowledgement signal isNACK, the radio base station 310 determines that the communication stateof the user data is poor. Alternatively, when the transmission powerused in the user-data communication exceeds a threshold value, the radiobase station 310 determines that the communication state of the userdata is poor. Alternatively, when the modulation coding scheme used inthe user-data communication is below a threshold value, the radio basestation 310 determines that the communication state of the user data ispoor.

When a result of the determination is “YES”, the radio base station 310performs a process in step 140. When the result of the determination is“NO”, the radio base station 310 completes a series of processes.

In step 140, the radio base station 310 assigns a reception resource forreceiving the user data directly communicated between the UE 10A and theUE 10B.

In step 150, the UE 10A transmits user data (initial transmission) byusing the radio resource assigned in step 110. Similarly, the UE 10B andthe radio base station 310 receive the user data (initial transmission)from the UE 10A by using the radio resource assigned in step 110.

Alternatively, between step 140 and step 150, it is possible to providea step of reassigning, by the radio base station 310, to the UE 10A andthe UE 10B, the radio resource used in the user-data communication inthe D2D communication from among the D2D radio resources.

Furthermore, it should be noted that when the user data is notsimultaneously received, the radio base station 310 is not able toretransmit the user data. That is, it should be noted that when thecommunication state of the user data is good, the retransmission controlof the user data is not performed.

Secondly, with reference to FIG. 22, a description will be provided forthe case in which the UE 10 (the UE 10A) determines whether thecommunication state of user data in the D2D communication is poor. InFIG. 22, the same reference numerals are used to the processes equal tothose of FIG. 21.

As illustrated in FIG. 22, in step S115, the UE 10A determines whetherto allow the radio base station 310 to receive user data directlycommunicated between the UE 10A and the UE 10B (to allow the radio basestation 310 to perform simultaneous reception of the user data).Specifically, when the communication state of the user data directlyperformed between the UE 10A and the UE 10B is poor, the UE 10Adetermines to perform the simultaneous reception of the user data.

Specifically, when transmission power used in the user-datacommunication exceeds a threshold value, the UE 10A determines that thecommunication state of the user data is poor. Alternatively, when amodulation coding scheme used in the user-data communication is below athreshold value, the UE 10A determines that the communication state ofthe user data is poor.

When a result of the determination is “YES”, the UE 10A performs aprocess of step 120. When the result of the determination is “NO”, theUE 10A completes a series of processes.

In step 120, the UE 10A transmits the D2D control signal to the radiobase station 310. Furthermore, it is preferable that the D2D controlsignal indicates at least one of: a signal for requesting theretransmission control of the user data; a signal indicating that thetransmission power used in the user-data communication has exceeded athreshold value; and a signal indicating that the modulation codingscheme used in the user-data communication has been below a thresholdvalue.

It should be noted that, in the case illustrated in FIG. 22, when theD2D control signal is received, the radio base station 310 assigns areception resource for receiving the user data directly communicatedbetween the UE 10A and the UE 10B.

In the case illustrated in FIG. 22, the D2D control signal istransmitted by the UE 10A. However, the embodiment is not limitedthereto. The D2D control signal may be transmitted by the UE 10B.

(Operation and Effect)

In the first modification, when the communication state of the user datais poor, the radio base station 310 assigns the reception resource forreceiving the user data directly communicated between the UE 10A and theUE 10B. In other words, when the communication state of the user data isgood, the radio base station 310 does not assign the reception resourcefor receiving the user data directly communicated between the UE 10A andthe UE 10B. That is, when the communication state of the user data isgood, the reception resource for receiving the user data directlycommunicated between the UE 10A and the UE 10B may be used to receiveuplink data from another UE 10. Consequently, it is possible to usefullyuse the reception resource of the radio base station 310.

Other Embodiments

The present invention is explained through the above embodiment, but itmust not be assumed that this invention is limited by the statements andthe drawings constituting a part of this disclosure. From thisdisclosure, various alternative embodiments, examples, and operationaltechnologies will become apparent to those skilled in the art.

The embodiment has described the case in which two UEs 10 communicatewith each other in the D2D communication. However, the embodiment is notlimited thereto. Specifically, in the D2D communication, three or moreUEs 10 may communicate with one another.

In the first modification, the communication state of the user datadirectly performed between the UE 10A and the UE 10B is mainlydetermined on the basis of the transmission power used in the user-datacommunication or the modulation coding scheme used in the user-datacommunication. However, the embodiment is not limited thereto. Thecommunication state of the user data directly performed between the UE10A and the UE 10B may be determined on the basis of at least one of: ablock error rate; a packet error rate; whether QoS is satisfied; CQI;and a processing load of the UE 10A.

In the first modification, it is determined whether the communicationstate of the user data directly performed between the UE 10A and the UE10B is poor. However, the embodiment is not limited thereto. It may bedetermined whether the communication state of the user data directlyperformed between the UE 10A and the UE 10B is good. It should be notedthat, in such a case, “YES” and “NO” are reversed for the result of thedetermination of step 130 illustrated in FIG. 21. It should be notedthat, similarly, “YES” and “NO” are reversed for the result of thedetermination in step 115 illustrated in FIG. 22.

Particularly not mentioned in the embodiment, it is possible to providea program for causing a computer to execute each process performed bythe UE 10 (the UE 10A or the UE 10B). Furthermore, the program may berecorded on a computer-readable medium. By using the computer-readablemedium, it is possible to install the program in a computer.Furthermore, the computer-readable medium recording the program thereonmay include a non-transitory recording medium. The non-transitoryrecording medium is not particularly limited. For example, thenon-transitory recording medium may include a recording medium such asCD-ROM or DVD-ROM.

Alternatively, it is also possible to provide a chip configured by amemory for storing a program for performing each process performed bythe UE 10 (the UE 10A or the UE 10B), and a processor for executing theprogram stored in the memory.

Thus, the present invention includes various embodiments which are notdescribed herein. Further, embodiments and modifications described abovecan be combined. Therefore, the technical scope of the present inventionis defined only by the matters specifying the invention according toclaims based on the above description.

In addition, the entire content of U.S. Provisional Application No.61/664,951 (filed on Jun. 27, 2012) is incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to present invention, it is possible to provide a mobilecommunication system, a radio terminal, a radio base station, a mobilecommunication method, and a processor which perform a retransmissioncontrol of user data.

The invention claimed is:
 1. A mobile communication system, comprising:a first radio terminal; a second radio terminal; and a base station,wherein the first radio terminal directly transmits user data to thesecond radio terminal by device to device (D2D) communication, thesecond radio terminal transmits a first radio signal, the first radiosignal indicates whether or not the second radio terminal has receivedthe user data, the base station receives the first radio signal from thesecond radio terminal, the base station transmits a second radio signalto the first radio terminal in response to a reception of the firstradio signal, the second radio signal indicates whether or not thesecond radio terminal has received the user data, the first radioterminal receives the second radio signal form the base station, and thefirst radio terminal determines whether to retransmit the user data tothe second radio terminal on a basis of the first radio signal withoutuse of the second radio signal in response to a communication state ofthe D2D communication exceeding a threshold value.
 2. The mobilecommunication system according to claim 1, wherein, the second radioterminal transmits the first radio signal to the base station inresponse to the communication state being below a threshold value.
 3. Afirst radio terminal, comprising: a transmitter that directly transmitsuser data to a second radio terminal by device to device (D2D)communication, a receiver that directly receives a first radio signalfrom the second radio terminal, wherein the first radio signal indicateswhether or not the second radio terminal has received the user data, thereceiver further receives a second radio signal from a base station, thesecond radio signal indicates whether or not the second radio terminalhas received the user data, the first radio terminal further comprises acontroller that determines whether to retransmit the user data to thesecond radio terminal, and the controller determines whether toretransmit the user data to the second radio terminal, on a basis of thefirst radio signal without use of the second radio signal in response toa communication state of the D2D communication exceeding a thresholdvalue.
 4. A first radio terminal, comprising: a transmitter thatdirectly transmits user data to a second radio terminal by device todevice (D2D) communication, a receiver that directly receives a firstradio signal from the second radio terminal, wherein the first radiosignal indicates whether or not the second radio terminal has receivedthe user data, the receiver further receives a second radio signal froma base station, the second radio signal indicates whether or not thesecond radio terminal has received the user data, the first radioterminal further comprises a controller that determines whether toretransmit the user data to the second radio terminal, and thecontroller notifies the base station of whether to use to the secondradio signal for the determination.
 5. A mobile communication method,comprising: directly transmitting, by a first radio terminal, user datato a second radio terminal by device to device (D2D) communication;transmitting, by the second user terminal, a first radio signalindicating whether or not the second radio terminal has received theuser data, directly receiving, by the first radio terminal, the firstradio signal from the second radio terminal, receiving, by a basestation, the first radio signal from the second radio terminal,transmitting, by the base station, a second radio signal to the firstradio terminal, the second radio signal indicating whether or not thesecond radio terminal has received the user data, receiving, by thefirst radio terminal, the second radio signal from the base station,determining, by the first radio terminal, whether to retransmit the userdata to the second radio terminal on a basis of the first radio signalwithout use of the second radio signal in response to a communicationstate of the D2D communication exceeding a threshold value.
 6. Aprocessing system for controlling a first radio terminal comprising aprocessor, and a memory communicatively coupled to the processor, theprocessor configured to: directly transmit user data to a second radioterminal by device to device (D2D) communication, and directly receive afirst radio signal from the second radio terminal, the first radiosignal indicating whether or not the second radio terminal has receivedthe user data, further receive a second radio signal from a basestation, the second radio signal indicating whether or not the secondradio terminal has received the user data, and determine whether toretransmit the user data to the second radio terminal, wherein in thedetermination, the processor determines whether to retransmit the userdata to the second radio terminal, on a basis of the first radio signalwithout use of the second radio signal in response to a communicationstate of the D2D communication exceeding a threshold value.